German Patent 21,56,506 describes a monitoring system to detect if a web in a printing machine should tear. Such web breaks or web tears may cause substantial damage in the printing stations due to wrap-around of the torn sheet about a cylinder. This is particularly dangerous with respect to the last printing station located in advance of a dryer or the like. If the web should tear at that point, the torn end of the web may be pulled into the preceding printing station and wrap around a cylinder and, if the machine is an offset printing machine, would usually wrap around the blanket cylinder. This increases the effective diameter of the blanket cylinder and causes damage in the printing station.
The referenced patent also describes an arrangement in which, when a web break is sensed, two rollers or cylinders, termed "paper capturing rollers", can be moved towards each other. In normal operation, the paper web is guided without contact between these two rollers. When a break is sensed, the rollers are moved towards each other so that the torn web can wrap around one of the capturing rollers which, however, is resiliently journalled so that increase in its diameter will not cause damage. Preferably, such capturing rollers are located in advance of the dryer. It is customary to journal one of the rollers in a fixed position, whereas the other one is movable. The two rollers are elastically engaged against each other, thus permitting roll-up of a torn or broken web on one or both of the rollers without leading to damage, since one of the rollers at least can resiliently radially deflect. Even if the paper does not wrap itself about one or both of the capturing rollers, the capturing rollers prevent that the torn web is pulled back into the last one, or any one of the preceding printing stations and cause damage therein.
The arrangement, as proposed, has a disadvantage, namely that with increasing operating speed of rotary printing machines, the time available to capture the broken or torn web becomes less and less. Due to the mass of the capturing rollers, criteria involving stability of the system limit the speed with which the rollers can be moved against each other. The spring force in known apparatus, which also in part determines the capturing speed or capturing time, cannot be increased beyond certain limits.